使用 Reachability 检查远程主机的可用性是否明智？

Question

Is it wise to use Reachability Class(from Apple) to check for a remote host's availability ? say for example, www.google.com

or should I use

NSString *connectedString = [[NSString alloc] initWithContentsOfURL:[NSURL URLWithString:@"http://www.google.com/"]]; 

if ([connectedString length] != 0) // Host Available

Which is the best option since I've heard that Reachability is having bug with checking for host's availability ?

Answer 1

Here is a good way to check if host is reachable:

    NSURLResponse *response=nil;
    NSError *error=nil;
    NSData *data = nil;
    NSURLRequest *request = [NSURLRequest requestWithURL:your_url];

    data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

If host is offline you will receive some error code in error , nil in data and nil in response variables.
If host is online and responds there will be some response , data , and error==nil .

Answer 2

As mentioned by others, reachability detects changes in your hardware instead of the real availability of a server. After reading many posts, I came up with this code.

Here is a full implementation of a ReachabilityManager class that uses both Reachability and URLConnection to make sure that the connection is available or not. Please note that depends on the Reachability classes (I am using the Tony Million implementation but I am sure it works with the Apple one)

If you test this in the simulator and enable/disable your wireless connection you will see that it detects (sometimes takes a few seconds) the connection/disconnection. Something that before with just the reachability class did not work.

I have also added a couple of notifications that are more effective than the ones from Reachability for the rest of your code. You may want to handle this in a different way.

Also, this is a singleton so you can instantiate if from anywhere. You can convert it to a non static class and instantiate it from the AppDelegate.

If a reader has time to create some UnitTests for it, let me know, so we can share more knowledge around the Reachability.

Just create a new NSObject class and copy this contents:

.h

#import <Foundation/Foundation.h>
@interface ReachabilityManager : NSObject

+ (void)startReachabilityWithHost : (NSURL *)hostName;

@end

.m

#import "ReachabilityManager.h"
#import "Reachability.h"

@implementation ReachabilityManager

static ReachabilityManager *_sharedReachabilityManager;
static Reachability *reachability;
static NSURL *_hostName;
static BOOL isServerReachable;
+ (void)initialize
{
    static BOOL initialized = NO;
    if(!initialized)
    {
        initialized = YES;
        _sharedReachabilityManager = [[ReachabilityManager alloc] init];
    }
}

+ (void)startReachabilityWithHost : (NSURL *)hostName{
    _hostName = hostName;

    reachability = [Reachability reachabilityWithHostname:hostName.host];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(reachabilityChanged:)
                                                 name:kReachabilityChangedNotification
                                               object:nil];
    [reachability startNotifier];
}

+ (void)stopReachability{
    [reachability stopNotifier];
    [[NSNotificationCenter defaultCenter]removeObserver:kReachabilityChangedNotification];
}

+(void)reachabilityChanged: (NSNotification *)notification{
    dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        BOOL isServerCurrentlyReachable = NO;
        do{
            isServerCurrentlyReachable = [self checkConnectivityToServer];
            BOOL wasServerPreviouslyReachable = isServerReachable;
            isServerReachable = isServerCurrentlyReachable;

            if (NO == wasServerPreviouslyReachable && YES == isServerCurrentlyReachable)
            {
                NSLog(@"REACHABLE!");
                [[NSNotificationCenter defaultCenter]postNotificationName:@"kNetworkReachabilityCustom" object:[NSNumber numberWithBool:YES]];
            }
            else if (YES == wasServerPreviouslyReachable && NO == isServerCurrentlyReachable)
            {
                NSLog(@"UNREACHABLE!");
                [[NSNotificationCenter defaultCenter]postNotificationName:@"kNetworkReachabilityCustom" object:[NSNumber numberWithBool:NO]];
            }
            [NSThread sleepForTimeInterval:5.0];
        }while(!isServerCurrentlyReachable);
    });

}


+(BOOL)checkConnectivityToServer{
    NSURLResponse *response;
    NSError *error=nil;
    NSData *data = nil;
    NSURLRequest *request = [NSURLRequest requestWithURL:_hostName];

    data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

    return (data && response);

}


@end

Answer 3

Reachability will not tell you if a remote host is contactable. It only tests the first hop ie can you send a packet to your router. If the router cannot reach the wider internet, reachability will still tell you that you have a wifi connection. You have to implement one of the other suggested solutions to test "true" reachability.

Answer 4

It is wise to check if you have any internet connection first, and for that I use Reachability. And I try to do my connection with the server only I have internet.

Answer 5

Here is a tutorial on how to use Reachability. http://www.raddonline.com/blogs/geek-journal/iphone-sdk-testing-network-reachability/

NSString *connectedString = [[NSString alloc] initWithContentsOfURL:[NSURL URLWithString:@"http://www.google.com/"]]; 

if ([connectedString length] != 0) // Host Available

The code you provided should also work but might not give the desired result.. Reachability is more reliable..

Answer 6

The following code from developer.apple.com might help you..

// Create the request.

NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:@"http://www.apple.com/"]

                        cachePolicy:NSURLRequestUseProtocolCachePolicy

                    timeoutInterval:60.0];

// create the connection with the request

// and start loading the data

NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];

if (theConnection) {

    // Create the NSMutableData to hold the received data.

    // receivedData is an instance variable declared elsewhere.

    receivedData = [[NSMutableData data] retain];

} else {

    // Inform the user that the connection failed.

}